It's OK if you're using 'a true gem' metaphorically to mean a very fine piece of jewelry. , a diamond, an emerald, a ruby). CJ
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It's OK if you're using 'a true gem' metaphorically to mean a very fine piece of jewelry.
Otherwise, you'd say that the pendant was set with a true/real [gem / gemstone] (e.g., a diamond, an emerald, a ruby).
CJ